Albert Barnes Commentary 2 Chronicles 21:1

"And Jehoshaphat slept with his fathers, and was buried with his fathers in the city of David: and Jehoram his son reigned in his stead." — 2 Chronicles 21:1 (ASV)

Jehoram’s sole reign now began (see the note on 2 Kings 8:16). His eight-year reign (2 Chronicles 21:5) must be counted from the time of his association with his father, which began in his father’s 23rd year.